WL#1039: Background updating of fulltext indexes

Full-text search is used for "fuzzy" matches in most cases, so it is not
necesary for updates to be indexed directly. We might wish to have the feature
of delayed index update for full-text search index, as well as manual update
(i.e. user runs daily update at night).
This would solve problems with very slow inserts for large texts, which is what
makes our mail archive system suffer a lot.